1. Statement of the Technical Field
The invention relates generally to Electronic Article Surveillance (“EAS”) systems, and more particularly to method for reduction of the backfield in EAS pedestal antenna systems.
2. Description of the Related Art
Electronic article surveillance (EAS) systems generally comprise an interrogation antenna for transmitting an electromagnetic signal into an interrogation zone, markers which respond in some known electromagnetic manner to the interrogation signal, an antenna for detecting the response of the marker, a signal analyzer for evaluating the signals produced by the detection antenna, and an alarm which indicates the presence of a marker in the interrogation zone. The alarm can then be the basis for initiating one or more appropriate responses depending upon the nature of the facility. Typically, the interrogation zone is in the vicinity of an exit from a facility such as a retail store, and the markers can be attached to articles such as items of merchandise or inventory.
One type of EAS system utilizes acousto-magnetic (AM) markers. The general operation of an AM EAS system is described in U.S. Pat. Nos. 4,510,489 and 4,510,490, the disclosure of which is herein incorporated by reference. The detection of markers in an acousto-magnetic (AM) EAS system by pedestals placed at an exit has always been specifically focused on detecting markers only within the spacing of the pedestals. However, the interrogation field generated by the pedestals may extend beyond the intended detection zone. For example, a first pedestal will generally include a main antenna field directed toward a detection zone located between the first pedestal and a second pedestal. When an exciter signal is applied at the first pedestal it will generate an electro-magnetic field of sufficient intensity so as to excite markers within the detection zone. Similarly, the second pedestal will generally include an antenna having a main antenna field directed toward the detection zone (and toward the first pedestal). An exciter signal applied at the second pedestal will also generate an electromagnetic field with sufficient intensity so as to excite markers within the detection zone. When a marker tag is excited in the detection zone, it will generate an electromagnetic signal which can usually be detected by receiving the signal at the antennas associated with the first and second pedestal.
It is generally desirable to direct all of the electromagnetic energy from each pedestal exclusively toward the detection zone between the two pedestals. As a practical matter, however, a certain portion of the electromagnetic energy will be radiated in other directions. For example, an antenna contained in an EAS pedestal will frequently include a backfield antenna lobe (“backfield”) which extends in a direction which is generally opposed from the direction of the main field. It is known that markers present in the backfield of antennas associated with the first or second pedestal may emit responsive signals, and create undesired alarms.
Several techniques have been implemented in the past to eliminate alarms causes by the backfield. One approach involves configuring the antenna in each pedestal in a manner which minimizes the actual extent of the backfield. Other solutions can involve changing from the traditional dual-transceiver pedestal to a TX pedestal/RX pedestal system, alternating TX/RX modes, and physical shielding of the antenna pedestals. A further approach involves correlating video analytics with marker signals. An ideal solution to the backfield problem is one which does not alter the detection performance of a system in a negative manner. For instance, although a system in which only one pedestal transmits and the other pedestal receives can reduce undesired alarms, pedestal separation in such a system must be reduced to accomplish the desired backfield reduction.